It has long been known (see Habashi F. and Dugdale R., Metall, 28, 129 (1974) -- Reference 1) that a variety of copper sulphites, including cupric-cuprous sulphite (Chevreul's salt), can be precipitated from solutions of cupric sulphate, using sulphur dioxide, sulphurous acid or soluble salts of sulphurous acid as the source of sulphite ion. Copper sulphites have a variety of stoichiometries and some contain both copper (II) and copper (I), so it is difficult to define them other than as those salts which are precipitated from copper salt solutions in water by addition of soluble salts of sulphurous acid, which contains sulphite ions. But all are potentially useful intermediates in copper processing. This observation has proved difficult to utilize in the extractive metallurgy of copper, because effective reduction of copper sulphites to metal has required elevated pressures and temperatures well above 100.degree. C (see reference 1 above and Arbiter N., Milligan D., and McClincy, R., I, Chem E. Symposium Series No. 42, 1.1 (1975) -- Reference 2). Reduction is often incomplete. Recent work has shown that cuprous ammonium sulphites can be precipitated from copper ammine salt solutions obtained by the oxidative ammonia leaching of chalcopyrite (see Reference 2 above). These were converted to copper metal by heating in an autoclave at 150.degree. C and 150 psig. It has also been shown that solutions of cuprous sulphate in water containing per mole of cuprous ion at least 3 moles of certain organic nitriles, notably acetonitrile and 2-hydroxycyanoethane, can be disproportionated either thermally (if acetonitrile or other volatile nitriles) to give particulate copper and cupric sulphate solutions or electrochemically (if acetonitrile or 2-hydroxycyanoethane) to give copper cathodes and cupric sulphate solutions, i.e. Cu.sub.2 SO.sub.4 .fwdarw. CuSO.sub.4 + Cu. Solutions of cuprous sulphate in water containing organic nitriles have considerable value as a source of copper (see Parker A. J., Search, 4, 426 (1973) -- Reference 3). Thus a method of converting cupric sulphate and slightly soluble copper sulphites to solutions of cuprous sulphate in water containing organic nitriles, has useful applications.